Packet forwarding is a process of relaying packets from one communication link to another by nodes in a network. A packet is a formatted unit of data that typically contains control information and payload data. Control information may include: source and destination IP addresses, error detection codes like checksums, sequencing information, etc. Control information is typically found in packet headers and trailers, with payload data in between.
At network nodes where multiple outgoing links are available, the choice of link to use for packet forwarding requires a decision process that, while simple in concept, can be complex. Since packet forwarding decisions are handled by nodes, the total time required for this can become a major limiting factor in overall network performance.
Label Distribution Protocol (LDP) is a mechanism where network nodes (e.g., routers) capable of Multiprotocol Label Switching (MPLS) exchange labels with their neighbors. These labels can be subsequently attached to packets and used by nodes to make packet forwarding decisions. For purposes of explanation only, nodes that make packet forwarding decisions based on labels will be referred to as LDP nodes. Packet forwarding based on labels stands in stark contrast to traditional Internet Protocol (IP) routing in which packet forwarding decisions are made using IP addresses contained within packets. Because LDP nodes use labels, there is no need for LDP nodes to examine the contents (e.g., IP addresses) of packets to make packet forwarding decisions.